Single crystals



April 1969 H. J. BORCHARDT 3,437,432

SINGLE CRYSTALS Filed July 21. 1966 Sheet of 2 F I G 1 30- 2 3'3MoO Host crystal 5 2 -3Mo0 Containing IO mgl Eu2O3 3Moo3 60- Dooed cr stal 5 "I E 40- II, C E 20- yo l l l l I l l I 1 I Wavelength, m,u

INVENTOR Hans J. Borchurdt April 1969 H. J. BCI-DRCHARDT 3,437,432

SINGLE CRYSTALS Filed July 21, 1966 Sheet 3 of 2 FIG.3

SYMBOLS SUPERIMPOSED IN 2 PHASE REGIONS l- LO2(MOO4)3 8O INVENTOR HANS J. BORCHARDT ATTORNEY United States Patent 0 3,437,432 SINGLE CRYSTALS Hans J. Borchardt, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Continuation-in-part of application Ser. No. 418,206, Dec. 14, 1964. This application July 21, 1966, Ser. No. 570,121 The portion of the term of the patent subsequent to June 1, 1982, has been disclaimed Int. Cl. C0lg 39/00 US. C]. 23-51 16 Claims ABSTRACT OF THE DISCLOSURE Stable single crystals having the gadolinium molybdate structure and unit cell dimensions wherein a and b are equal to l0.39i0.07 A. and C is l0.69i0.09 A. said crystals having one dimension at least 0.01 mm. and represented by the formula (R R' O -3Mo W O wherein R and R represent at least one rare earth element having atomic number of from 57-71, scandium and yttrium, x is from 0 to 1. Oand e is from 0 to 0.2. The crystals are made by conventional single crystal techniques and have fluorescent and ferroelectric properties. The crystals are useful as ceramic insulators, as filters for screening out ultraviolet radiation, as laser materials, as transducers, and as ultrasonic generators.

This application is a continuation-in-part of application Ser. No. 150,477, filed Nov. 6, 1961, now abandoned, application Ser. No. 186,602, filed Apr. 6, 1962, now Patent 3,250,722, application Ser. No. 264,065, filed Mar. 11, 1963, now abandoned, and application Ser. No. 418,- 206, filed Dec. 14, 1964, now abandoned.

The present invention relates to single crystals of Gd O -3MoO and single crystals having the same crystal structure as single crystals of Gd O -3MoO wherein other atoms have been substituted for at least a portion of the gadolinium or molybdenum or both.

Known monocrystalline bodies of rare earth compounds are very few in number. Those rare earth compounds which do grow in single crystals generally possess disadvantages which make them unsuitable for electrical or optical uses. For example, yttrium tungstate and yttrium molybdate, do grow as single crystals but these crystals are not stable and hydrate when exposed to moist air, thus severely limiting their usefulness. Efforts to grow lanthanum tungstate, lanthanum molybdate and gadolinium tungstate have failed.

Another group of rare earth containing crystals is the substituted scheelites. These are materials such as CaWO and CaMoO where a portion of the calcium is replaced by a rare earth. Such materials are relatively easy to grow in single crystal form, partly because of their symmetrical tetragonal structure. However,they are limited in three ways by having a divalent cation sublattice which normally accommodates Ca:

(1) Only partial substitution of trivalent rare earth element for divalent calcium is possible and this only by inclusion of a charge compensator such as an alkali metal "ice (monovalent) which dilutes the rare earth concentration.

(2) The charge compensator and rare earth ion tend to pair causing the rare earth ion to be in a nonsymmetrical environment. This affects the optical properties of the rare earth ion.

(3) During crystal growth, segregation tends to occur in such multicomponent systems and often results in a nonuniform distribution of rare earth ion within the crystal.

A need exists for single crystals of rare earth compositions because of the usefulness of such crystals as contrasted to powdered materials.

It has been discovered that gadolinium molybdate, i.e., Gd O -3MoO or alternatively expressed Gd (MoO or compositions having the gadolinium molybdate structure do grow in the form of stable, e.g., nonhygroscopic, single crystals. It has further been found that all or a part of the gadolinium can be replaced by rare earth ions which form isostructural molybdates and part of the gadolinium or other ion which forms isostructural molybdates can be replaced by other rare earth ions having ionic radii close to that of gadolinium without substantially changing the crystal structure.

The single crystals of the present invention have the gadolinium molybdate structure and unit cell dimensions wherein a and b are equal to 10.39i0.07 A and c is 10.69i0.09 A., said crystals having one dimension at least about 0.1 mm. and represented by the formula:

wherein R and R are at least one rare earth element, 2 is from about 0 to 0.20 and x is from 0 to 1. The term rare earth element as used herein includes scandium and yttrium and elements having an atomic number of 5 7 through 71.

More particularly, the preferred stable single crystals of the present invention have the gadolinium molybdate structure with an orthorhombic unit cell wherein the dimensions of a and b are equal to l0.39i0.07 A. and c is l0.69:0.09 A., said crystals having one dimension at least about 0.1 mm. and having a composition represented by the formula:

. or dysprosium, B, C, D and E are the same as A or scandium, yttrium, lanthanum, cerium, raseodymium, neodymium, holmium, erbium, thulium, ytterbium or lutetium; when B, C, D and E are selected from the group represented by A, x, y, z and d can range from 0 to 1, the sum of x+y+z+d being not greater than 1; when B, C, D or E is other than A the values of x, y, z and d are, of course,, limited to those values giving a gadolinium molybdate structure, i.e., unit cell dimensions for the gadolinium molybdate srtucture are 103910.07 A. in the a and b direction and 10.69i0.09 A. in the c direction, the sum of x, y, z and d being not greater than 1; and e is from 0 to 0.20. For example, FIGURE 3 illustrates a ternary composition diagram of the system 2( 4)a' 2( 4)s- 2( 4)3 It can be seen from the diagram that where either or both d (A.) III TABLE II [SmrOz-3Mo0z] 10.45*0.01 A; b=10.450.01 A.; C

TABLE III [EllzO -3M0O3] 1o.42=o.01 A.; b=10.42= =0.01 A.; c

TABLE IV 'rmol-snoon f compositions. Compositions that are especially preferred are those wherein A is europium, gadolinium, terbium or dysprosium or mixtures thereof.

3,437,432 3 n I yttrium and lanthanum are substituted for gadolinium, the Gd (MoO structure is stable over a wide range 0 The term single crystal as employed in the present 5 invention refers to orthorhombic crystalline material in which the single crystals have gross physical dimensions such that at least one dimension of the crystal is at least 0.1 millimeter. A single crystal for the purposes of this invention is one which satisfies the well known, external morphological, optical and X-ray criteria for singularity. The criterion of singularity with respect to domain structure is not a mandatory specification. The single crystals are all substantially homogeneous although small imperfections may exist without impairing their utility. This means that x is such a value that no second phase separates. The single crystals of the present invention are nonhygroscopic and can be grown to a length of 4 cm.

or more with a diameter of 1 cm. or more. Weissenberg and precession X-ray photographs of single crystals of Gd O -3MoO can be indexed on the basis of a tetragonal unit cell having cell constants as follows: a and b:

Lattice constants: a

10.39:.07 A. and c=l0.69' -.09 A. A strong piezoelectric signal is observed from these crystals, e.g., a 25 crystal about 1 x 2 x 3 mm. in size, showing that the structure is not centro symmetric and that the true symmetry is not tetragonal but orthorhombic and the space group is Pba2. This X-ray data and that presented subsequently were all determined at room temperature. The gadolinium molybdate single crystals have a melting point of 1180 Gill). The experimentally determined density is 4.52 g./cm. (theory for the observed unit cell 1 60626 32226 m w u mi 2111 =4.59 g./cm.

SIIIZO3'3MOO3, EUZO3'3MOO3, Tb203'3MO03 and Dy203'3MOO 3 are isostructural with Gd O -3MoO and the lattice constants as calculated from powder patterns, given below, have the same unit cell dimensions as the single crystal 40 compounds. These compounds all have the Gd O -3MoO structure and come within the critical unit cell dimensions of said structure as defined in the application.

TABLE I Sample a (A.)

The rare earth compounds and solid solution of this invention can be characterized by conventional X-ray diffraction techniques and composition. X-ray spectra can LLLLLl LLl Lllllllll Lattice constants: a

be conveniently determined on a Norelco X-ray ditfrac tometer, CuK, radiation, 1 slits, a nickel filter and a scan rate of 1 of 20/min. If greater resolution is desired,

a Guinier camera can be employed. The X-ray patterns of the compositions of this invention are characteristic thereof and are diiferent from the patterns of the reactants leading to their formation. For instance, it is 0 significant to note that the X-ray diffraction pattern for Gd O -3MoO is different from, and not the summation of, the patterns for Gd O and M00 The novel compositions of the present invention are single crystals of chemical compounds and solid solu- 5 tions of chemical compounds as shown by X-ray diffraction spectra and not mixtures of the rare earth oxides with molybdic oxide. The complete X-ray powder diflfraction patterns for several of the compositions of this invention having the Gd O -3MoO structure as defined in the application are given in Tables II through VI. These diffraction patterns are considerably different from patterns of M00 Gd O or mixtures thereof. When an excess of any one of the constituents used in making the compound is mixed with the compound, the X-ray difiFrac- TABLE IV-Continued d (A.) I/Io d (A.) I/In d (A.) I/I Lattice constants: a =10.35=l=0.01 A.,; b=10.35=l=0.01 A.; c= 10.634001 A.

TABLE V [Dy;Oa-3Mo0:l

d (A.) I/In d (A.) III (1 (A.) I/I Lattice constants: a=10.33 0.01 A4 b=10.33* 0.01 11.; c=10.61*0.01 A.

TABLE VI [Gd203-3M0O3] d (.4.) I/I d (4.) III. d (A.) I/Io Lattice constants: a=10.38i0.01A.; b=10.38;t;0.01A.; c=10.69:1:0.01A.

The space group and unit cell dimensions characterize the structure of the crystals which are the subjects of this invention. The unit cell dimensions have a range of values dependent in part upon the size of the cation(s) employed. If the cation is too large or too small a different structure will result. The Ahrens ionic radii of the rare earth ions of atomic numbers 60 to 67 are as follows: Nd+ 1.04 A. (Pm does not occur naturally), Sm+ 1.00 A., Eu 0.98 A., Gd+ 0.97 A., Tb 0.93 A., Dy 0.92 A., and Ho 0.91 A. It is found that the compounds R O -3MoO where R is samarium, europium, gadolinium, terbium and dysprosium are isostructural, all having the Gd O -3MoO structure, whereas the cognate compounds of neodymium and holmium have diiferent structures. Thus, the range of unit cell dimensions of the single crystals having the Gd O -3MoO structure which are the subject of this invention are defined as those having unit cell dimensions where a and 'b are equal to l0.39i0.07 A. and c is 10.6-9:t0.09 A.

In the substitution of ions other than those represented by A for gadolinium, a combination of ions, one smaller and one larger than gadolinium can be used to insert a larger quantity of substituent ions into the Gd O '3MOO' structure. In addition to the ions mentioned hereinbefore small quantities of other trivalent ions (up to about 1%) such as gallium and aluminum can be substituted for the gadolinium without substantially distorting the lattice. Also oxygen can partially replace another ion. Substitution of such ions is contemplated by this invention. Also ions of valence other than three can be substituted provided that the electrical neutrality of the system is maintained by incorporation of other ions to make the average valence three.

The compounds of this invention are prepared by mixing materials comprising a rare earth component and a component which contributes M00 and, when desired, W0 and thereafter heating the resulting mixture at elevated temperatures; alternately, the preformed rare earth molybdates can be mixed and heated at elevated temperature. A reaction temperature of at least about 700 C., and usually at least about 900 C., is employed. However, since the reaction time decreases as the reaction temperature increases, to insure complete reactions in practical periods of time, high reaction temperatures approaching, e.g., within C. of, but in any case below, the temperature at which localized fusion of the reaction mass begins are preferred. If relatively low-melting eutectics are formed locally during the reaction, it may be desirable to' heat the reaction mixture for a period at, e.g., 500-600 C., then regrind the resulting product and finish the reaction at higher temperature.

The rare earth components preferably are introduced into the reaction mixture as oxides. However, rare earth components which decompose to the oxide on heating, for example, rare earth hydroxides, oxalates, carbonates, citrates, acetates and tartrates can be employed. The W0 or Moo -contributing component also is preferably introduced into the mixture in the form of an oxide, such as tungsten oxide or molybdenum oxide. However, this reactant need not necessarily be an oxide. It can be, for example, a compound such as tungstic acid, or molybdic acid, or ammonium tungstate, which on ignition is converted to the oxide. Reactants are preferably introduced in the form of finely ground particulate material, preferably having a particle size of less than 10 microns.

The quantities of reactants employed in preparing the compositions of this invention are preferably approximately stoichiometric based on the desired composition of the final product. Thus, for example, in preparing Gd O -3MoO three moles of M00 would be heated with one mole of Gd O After the powdered materials are prepared they are heated to form a melt and the single crystals having the gadolinium molybdate structure are pulled from the melt by conventional methods. The techniques used for pulling the novel single crystals from the melt are described in Preparation of Single Crystals, Lawson and Nielson, Butterworth, 1958, particularly pp. -31 and 176-210. Because the crystals are grown from a homogeneous melt they have substantially the same composition as the melt.

The single crystals of the present invention are useful as ceramic insulators and as filters for screening out ultraviolet radiation. The near ultraviolet and visible spectra of two single crystals are shown in FIGURE 1. This figure shows that absorption of ultraviolet light is substantially complete in the region from 2000 to about 3200 A.

As exemplified hereinafter, some of the single crystals of the present invention are also useful as laser materials. As is well known, the acronym, laser, means light amplification through stimulated emission of radiation and numerous references are available which describe the principles and operation of lasers.

The single crystals of the present invention have been demonstrated to possess ferroelectric properties. Ferroelectric properties of several crystals are given in Table VII below. The procedures for measuring the ferroelectric properties are described and referenced in Applied Physics Letters, vol. 8, No. 2, pp. 50-52, Jan. 15, 1966. The discovery of materials which exhibit both laser and ferroelectric properties is very surprising. This combination of properties in a single material is novel.

The single crystals of the present invention are also useful because of their piezoand ferroelectric properties, as trandsucers and ultrasonic generators.

8 All the single crystals of the present invention are useful as ceramic insulators. The electrical properties of the single crystal of Gd O -3MoO are shown below:

TABLE VIII.PROPERTIES 0F GdzOs-3M0Os Compound (Gd O -3MoO Resistivity (ohm/cm.) 5 X10 Dielectric constant 8.8 Dissipation factor 0.80

EXAMPLE 1 Gd O (361.8 parts) is intimately mixed with M00 (431.7 parts). About 1% paraffin is added as a binder and the mixture pressed into pellets. These pellets are heated in air in a platinum vessel for 4 hours at 900 C. During the heating the paraffin is burned away. The mixture is converted into the compound Gd O -3MoO for which the X-ray diffraction pattern is given in Table VI.

A platinum-rhodium crucible (-40) measuring 1 /2 x 1 /2 inches (0.060 inch-thick wall) located in an electrical induction coil which will carry 10 kilowatts at 450 kilocycles is half filled with Gd O -3MoO prepared as above which is melted at approximately 1185 C. A platinum-rhodium wire (-20) 0.045 inch diameter is dipped into the melt and rotated at approximately RARE EARTH MOLYBDATES P., Micro- Eu, Kilo- Crystal Composition T., C. T., C. coulornbs/ volts} cm.

1 Gd203-3M003 159 25 17 5. 0

2 TbzOs-3M0Oz 157 25 18 G. 3 100 13 5. 2

3 EUIO3-3MOO3 161 25 12 9. 4 100 10 10. 7

4 SI112Oa-3M003 190 25 100 19 8. 5

5 (Gdo.z5Tbo.7s)zOa-3M00a...-- 157 25 24 8. 0 100 10 3. 4

6 (Gd YM) 20a-3MOOs.....- 147 25 21 11. 5 100 13 7. 5

7 (Gdo.sEllo.2) :O3'3M0Oa 25 14 9. 5 100 10 5. 8

8 (GdogElloxhOa-iiMOOg-. 157 25 22 6.9 100 11 3. 8 140 07 1. 7

9 (Tba vEuu.1)1Os-3Mo0; 159 25 .15 8. 8 100 U9 4. 4 150 U5 1. 8

10 (Gdu n7Ndo.ns):0a-3M0Oa 159 25 17 10. 5 100 14 13. 3 140 09 11. 6

11 (Tbo 5Eu0,5)1033M0O3-.....- 25 08 6. 6 100 O9 7. 3

12 Gd3O3-3(M00.!5W0.15) 03"..-. 148 25 13 7. 5 100 13 4. 3

To Curie temperature. P.=Spontaneous polarizations. E.=Coercive fields.

1'.p.-m. until solidification on the wire occurs. The wire is then withdrawn from the melt at about /3 inch per hour. The power to the induction coil is increased until the crystal diameter is approximately 1 mm. The power is 10 fluorescence at 77 K. centered at 5800 A. (half-width 600 A.) with a 2537 A. exciting source.

A similar crystal prepared in substantially the same manner, but containing 10 mole percent europium, gives then decreased until the crystal diameter is about to 5 two strong doublets at about 6150 A. and 6170 A. At /2 inch. The power is then held such that the crystal room temperature these doublets each form one band maintains a dameter of /8 to /2 inch as it is being with 8-15 A. half width. At liquid nitrogen temperatures drawn at /3 inch per hour. This drawing is continued the doublets are well defined with half-widths of about 1 until the crystal is about 2 inches long. To remove the A. for each line. crystal from the crucible, a split tube furnace 10 inches 10 The absorption of the crystals is measured with a long by 1 inch in diameter is preheated to approximately Bausch and 'Lomb Spectromic 505 spectrophotometer. 400 C. and placed immediately above the crucible and The Gd O -3MoO single crystal has strong absorption around the pulling rod to which the wire is attached. The below 3200 A. but little absorption above. The europium crystal is pulled rapidly (about 6 inches/minute) into the doped single crystal also has strong absorption below middle of the furnace. The voltage to the furnace is pro- 3200 A. as well as in several bands between 3200 and grammed downward from about 60 volts to 0 linearly 6 200 A. as shown in FIGURE 1. over approximately 5 hours. The crystal is then removed. The reflectance spectrum of a Gd 'O -3MoO powder T fluores e ce of a G 1 -3h I crystal i meas. is measured with a Beckmann spectrophotometer using a med with a Bauseh and Lomh spectrograph producing Beckmann reflectance attachment on a USP magnesium plates with a dispersion of 8 A./mm. and a spectral resocarbonate black as reference mflterlal- T e reflectance is lution of 0.1 A. The fluorescence is measured with ex- 10W between 2000 and 2 800 but Increases rapldly citing wave lengths of 2537 A. and 3600 A. both at after 2800 as shwn FIGURE room temperature and immersed in liquid nitrogen (77 EXAMPLES 2 THROUGH 54 K.)- The 3600 exciting source is a Gates and The procedure described in Exam le 1 is used for re- HEO 1 0 V\ 2 g Pressure f lamp elumped paring the following single crystaih. The amounts of with a Cor lng 5840 fi f 3 P 0f P the reactants shown in the following table are heated ing glass. The 2537 A. exciting source is an Ultra-Violet to just below the fusion point of the compound to P o uc y p used With a coming 9863 filter form a solid solution. Where terbium is one of the conand a 1.8 cm. chlorine gas cell. Exposures with this latter s itu nt it was dd d as Tb Q., U d h influence of source require several hours while the high-pressure heat this form is converted to Tb O The order of the mercury lamp requires only a few minutes. Following amounts of the reactants is the same as the formula development the plates are analyzed with a Jarrett-Ash t0 the left side of the table. Then the compounds are densitometen further heated to from a melt and the single crystals The Gd O -3MoO crystal gives a white broad-band are pulled from the melt as described in Example 1.

Single Crystal M00; Rare Earth Oxides TbzOa-3M0Os Gd0 Sm0.2 y0.l-3M0Og (Euo.wNdo.1)2 a-3M0Oa-- 3 (Tbo.n7Ndo.o:)20a-3M0Oa (Gdu.uYo.sLau.o5P1o.os)2Oa'3MoOi (Tbn.vs b.os)2 s-3M0o.aoWuoOa dm os aum ru. :0:- 00a... 0.5T 0.3 0.2 Y0.1)2 3 M0 1 o.a bo.: 11o.2 Yo.1 o.1) 20:-3M001--.. 0.5 1 0.|H0o.1)2 a-3M0u,oWn.1 3 0.s 1 o.4 o.1)2 s-3M0Oa EGdmTbosTmnsYbnJ)2 a-3M0 a Gdo.95 o.u5)203-3Mo a o.v1 o.os)20a-3MO (Tbo 9s o.os)z s 0 39. 4 (Gdu gTbogElJmsNdu 1) 31V 224. 6 211.2 87. 2 oa osEuos moa)20a 3M0 217. 1 224. 6 211. 2 77. 4 o.a o.4Ybo.i)2 s-3M0Os.. 863. 6 374. 4 281. 6 78 8 (Gdom ms) zOa-3M0O 431. 8 343. 7 19. b0.o5 0o.oa)2 r a- 863. 6 711. 4 37. (TbwYbn .o:)2Oa-3M0 s 863. 6 711. 4 39. 05 81051100.1111)1 a' 0 s 431. 8 217. 1 67. 863. 6 361. 8 149. 863. 6 711. 4 39. 431. 8 343. 7 18. 863. 6 711. 4 38. 431. 3 343. 7 16. 431. 8 217. 1 67. 863. 6 711. 4 34. 431. 8 217. 1 b7. 863. 6 711. 4 33. (Tb Yu .3L8o.osDYO .115) 20s-3M0O5 863. 6 449. 3 135. MYo.a flu.o5 l1u.os)2 a- 0 431. 8 217. 1 67. u .t o.aL8o.0aS n.o5) 20 a-3M001 431. 8 217. 1 67. o.oYu.a flo.o5 0.n1)203-3MO02 431. s 217. 1 e7. (Gdo.uYn .3L80.115Tb0.05) 2O -3M0O3 863. 6 434. 2 135. 0.s bn.1 1 o.aS u.1) 2 a-3M00a 863. 6 217. 1 224. (Gdo,s'lbn aE110.aH00.1)20s-3M0O;- 863. 6 217. 1 224. (SmmYm) 20a-3MOO3 431.8 279.0 45. (Smo,nEIo,1)aO;-3M0Os 431.8 313.8 38.3 54 (Smu.sH0o.2)20a-3M00:-. 431. 8 479. 0 6

I cf. Figure 3. b Plus 46.4 parts 0! W03. Plus 23.2 parts 01 W01.

1 1 EXAMPLE 55.-DEMONSTRATION OF FERROELECTRICITY A crystal of Gd O -3Mo0 prepared as in Example 1, was cut to provide -a wafer circa 1 cm. in diameter and 0.04 cm. thick and electrodes 0.4 x 0.5 cm. were attached to it. The sample was oriented with its c-axis normal to the electrodes. It was tested in a ferroelectric loop tracer built after the design of Diamant et al. (Rev. Sci. Instr. 28, 30 (1957)) where a dielectric hysteresis loop was observed at room temperature at 60 c.p.s., establishing that the material is ferroelectric. The coercive field was found to be approximately 5000 v./cm. and the spontaneous polarization approximately 0.17 microcoulomb/cm. at room temperature. The values of these properties at two other temperatures are given in Table VII.

EXAMPLE 56.DEMONSTRATION OF LASING ACTION A single crystal having the composition o.91 o.03)2 a a (prepared as in Example 3) and a diameter of inch and a length of 1 inch is polished flat at both ends. A mirror is applied at both ends of the crystal by conventional techniques which is 100% reflecting at one end and 95% reflecting at the other. The crystal is then cemented in a brass holder with inch of the crystal extending beyond the holder.

The crystal and holder is then inserted in the head of a Trion model C-102-85 laser system using an FT-lOOB flashlamp. Emission of radiation from the 95 reflecting mirror end of the crystal is measured by an infrared photomultiplier in conjunction with an oscilloscope which gives a trace of intensity versus time.

Successively higher energies are flashed into the crystal. Up to 925 joules ordinary fluorescence of neodymium is seen. At 945 joules the first evidence of stimulated emission is seen and at 975 joules stimulated emission is very pronounced.

In a similar experiment in which the crystal is cooled to 134 K. in a current of cold nitrogen the threshold energy is 340 joules.

I claim:

1. Stable single crystals having the gadolinium molybdate structure and unit cell dimensions wherein a and b are equal to 10.39-10.07 A. and c is 10.69i0.09 A. said crystals having one dimension at least about 0.1 mm. and represented by the formula (R R' O -3Mo W O wherein R and R represent at least one rare earth element having an atomic number of from 57 through 71, scandium and yttrium, x is from to 1.0 and e is from 0 to 0.2.

2. The single crystal of claim 1 wherein R is gadolinium and R represents europium, dyprosium, scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, holmium, erbium, thulium, ytterbium, lutetium, and terbium.

3. The single crystal of claim 1 wherein R is europiurn and R represents dysprosium, scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, holmium, erbium, thulium, ytterbium, lutetium, and terbium.

4. The single crystal of claim 1 wherein R is dysprosium and R represents scandium, yttrium, lanthanum,

cerium, praseodymium, neodymium, samarium, holmium, erbium, thulium, ytterbium, lutetium, and terbium.

5. The single crystal of claim 1 wherein R is terbium and R represents scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, holmium, erbium, thulium, ytterbium, lutetium, and terbium.

6. The single crystal of claim 1 where R is samarium and R is scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, holmium, erbium, thulium, ytterbium and lutetium.

7. The single crystal of claim 1 wherein R is gadolinium and R is europium.

8. The single crystal of claim 1 wherein R is gadolinium and R is neodymium.

9. The single crystal of claim 1 wherein R is gadolinium and R is terbium.

10. The single crystal of claim 1 wherein R is europium and R is terbium.

11. The stable single crystal of claim 1, Gd O -3MoO 12. The stable single crystal of claim 1, Sm O -3MoO 13. The stable single crystal of claim 1, Eu O -3MoO 14. The stable single crystal of claim 1, Tb O *3MoO 15. The stable single crystal of claim 1, Dy O '3M0O 16. Stable single crystals having the gadolinium molybdate structure with an orthorhombic unit cell wherein the dimensions of a and b are equal to l0.39-' :0.07 A. and c is 106910.09 A., said crystals having one dimension at least about 0.1 mm. and having a composition represented by the formula wherein A is selected from the group consisting of samarium, europium, gadolinium, terbium, and dysprosium; B, C, D and E are selected from the group consisting of A, scandium, yttrium, lanthanum, cerium, praseodymium, holmium, erbium, thulium, ytterbium, and lutetium; when B, C, D and E are selected from the group represented by A, x, y, z and d are 0 through 1, the sum of x, y, z and d being not greater than 1; when B, C, D, and E is other than from the group represented by A the values of x, y, z and d are limited to those giving a gadolinium molybdate structure, the sum of x, y, z and d being not greater than 1; and e is from 0 to 0.2.

References Cited UNITED STATES PATENTS 3,152,085 10/1964 Ballman et al. 25230l.4 3,183,193 5/1965 Soden et al. 25230l.4 3,186,950 6/1965 Borchardt 25230l.4 3,243,723 3/1966 Van Uitert 25230l.4 3,250,722 5/1966 Borchardt 25230l.4 3,257,625 6/1966 Johnson et al. 25230l.4 3,294,701 12/1966 Vogel et al. 242301.6

OTHER REFERENCES Hoffman: Lexikon der Anorganischen Verbindungen, Band 2, Al-X No. 56-81, 1914, pages 625 and 747-751.

TOBIAS E. LEVOW, Primary Examiner.

ROBERT D. EDMONDS, Assistant Examiner.

US. Cl. X.R.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,437,432 April 8, 196

Hans J. Borchardt It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 12, line 36, "praseodymium, holmium," should read praseodymim neodymium, holmium Signed and sealed this 7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

